I was wondering how "Burstable Ram" works. I’ve purchased 5 GB of Ram for my VPS account, but was told that “the assigned RAM amount will change according to your server's needing. If your server does not require higher amount of RAM”.

However, I’ve been monitoring my website's memory usage because I’ve been experiencing some performance issues and downtime. I’ve noticed that once my site gets to 3 GB of ram, it starts using virtual memory. I was wondering why the “Burstable Ram” doesn’t kick in at that point? Obviously, I’d prefer my application to use the 5 GB of RAM I am paying for FIRST, and then use virtual memory. Thanks ahead of time for helping understand how this works.